The present invention relates generally to systems for detecting, counting and measuring step-like signal events produced by preamplifiers connected to radiation detectors used to detect x-rays, gamma-rays, nuclear particles, and the like. More particularly, it relates to increasing the accuracy with which the amplitudes of these step-like events are measured by increasing the accuracy of determining the event energy filter""s baseline output at times when no events are being processed. The specific embodiment described relates to a spectrometer used with a solid state detector, but the same techniques can readily be applied a to radiation spectrometers operating with other detectors as well, since their operating principles are nearly identical.
A Synopsis of Current Spectrometer Art
FIG. 1 is a schematic diagram of a prior art radiation spectroscopy system employed with a solid state detector diode 7. Similar systems are used for measuring x-ray, gamma-ray and alpha and beta particle radiations, differing primarily in the physical form of the detector diode 7, which might also be replaced with a proportional counter or other detector. All of these detectors 7 share the common property that, when biased by a voltage supply 8, they produce an output current pulse when detecting an absorption event and the total charge QE in this pulse is approximately proportional to the energy E of the absorbed ray or particle. This current flows into a preamplifier 10 having an amplifier 12 and a feedback capacitor 13. The current is integrated onto feedback capacitor 13 by an amplifier 12, whose output is then a step-like pulse of amplitude Ae=QE/Cf, where Cf is the capacitance of feedback capacitor 13. Other types of preamplifiers may process the current pulses to provide other types of output pulses. For example, a transimpedance amplifier would produce an output voltage pulse proportional to the input current pulse. As a matter of nomenclature, pulses produced by the preamplifier in response to events will sometimes be referred to as event pulses.
A spectroscopy amplifier 15 is then used to measure AE. Within modern spectroscopy amplifiers 15 the output of preamplifier 10 is typically sent to both a xe2x80x9cslowxe2x80x9d energy filter circuit 17 and a xe2x80x9cfastxe2x80x9d pileup inspection circuit 18. The energy filter circuit filters the Ae step to produce a low-noise shaped output pulse whose peak height AE is proportional to Ae. The pileup inspection circuit applies a filter and discriminator to the preamplifier output to inspect for the presence of Ae signal steps (events) and signals the filter peak capture circuit 20 to capture the amplitudes AE of shaped pulses from the energy filter circuit 17 which are sufficiently separated in time so that they do not interfere with each other""s amplitudes (i.e., do not xe2x80x9cpile upxe2x80x9d). The distinction between the xe2x80x9cfast and xe2x80x9cslowxe2x80x9d filters is relative, based on the particular application, but the xe2x80x9cfastxe2x80x9d filter""s time constant is typically an order of magnitude shorter than the xe2x80x9cslowxe2x80x9d energy filter""s time constant. A typical x-ray spectrometer, for instance, might use a 200 ns fast filter and a 4 xcexcs energy filter. The inspection circuit 18 also determines when events are sufficiently separated so that the output of the energy filter circuit has returned to its DC value and signals a baseline capture circuit 22 to capture these values so that they may be subtracted from captured peak values by a subtraction circuit 23. These differences are then passed to a multichannel analyzer (MCA) or digital signal processor (DSP) 25, for binning to form a spectral representation (spectrum) of the energy values present in the incident radiation.
It should also be noted that, although the most commonly implemented spectrometers capture the value of the shaped pulse""s amplitude AE as an estimate of the energy, other designs measure other values that characterize the shaped pulse. For example, some designs capture the value of the shaped pulse""s area, or capture several points across the shaped pulse and then fit a mathematical function to the captured points. As a matter of nomenclature, we shall refer to these various measured values as xe2x80x9ccharacteristic valuesxe2x80x9d of the shaped pulse. Because the energy filter is a linear filter, these characteristic values are all proportional to one another, although which has the best signal to noise properties will depend upon the nature of the energy filter and the noise spectrum of the detector-preamplifier combination. Therefore, although we shall primarily refer to the shaped pulse""s amplitude AE in the remainder of this specification, since it is the most commonly measured characteristic value of the shaped pulse, the reader should bear in mind that it is, in fact, only a representative of all the various characteristic values that could be measured and that our invention applies to them all as well.
The Need for Baseline Correction
FIG. 2A shows the need for baseline correction in radiation spectrometers of the type described above. Trace 30 represents the output from the preamplifier 10, with the step 34 occurring in response to detected radiation. As drawn, trace 30 has a slight slope, which physically may result from such causes as detector leakage current, a non-ideal amplifier 12, or noise pickup. Trace 32 represents the output of energy filter 17, which produces the shaped pulse 35 in response to input step 34. The actual amplitude A 37 of the pulse is seen to be the sum of the height of pulse 35 and the baseline (DC) offset B 38 of the signal, where this baseline offset results from the reaction of the filters in the energy filter circuit 17 to the slope in the preamplifier output signal 30. Thus, in the spectroscopy amplifier 15 shown in FIG. 1, the function of the peak capture circuit 20 is to capture the values A 37 in FIG. 2A, while the baseline capture circuit 22 measures baseline values B 38, and the subtraction circuit 23 outputs the difference A-B which represents the energy of the radiation absorbed in the detector 7.
The art of building spectroscopy amplifiers is relatively mature and many variations, using both analog and digital electronics, exist on the basic circuit shown in FIG. 1. The reference book by Knoll [KNOLL 1989] provides a good introduction to the subject. See, for example, Chapter 16, Section III, xe2x80x9cPulse Shaping.xe2x80x9d
In some designs the filter peak capture circuit senses and captures peaks autonomously and the job of the pileup inspector is to discriminate between valid and invalid captures and only allow valid values to pass on to the subtraction circuit 23. Indeed, even the order of the components shown may be altered to achieve the same ends. Thus, in analog circuits the baseline capture circuit is commonly a switched capacitor which is tied to the output of the energy filter circuit 17 as long as the baseline is valid and disconnected whenever the pileup inspection circuit detects an event. The time constant of this circuit is long enough to filter baseline noise. The subtraction circuit 23 is then typically an operational amplifier with the capacitor voltage applied to its negative input and the peak capture circuit 20 applied to its positive input. In some cases, however, the order of circuits 20 and 23 are reversed, so the offset is removed from the energy filter circuit""s output before peak amplitudes are captured. In traditional MCA""s, in fact, the peak capture capability is included in the MCA 25 and removed from the spectroscopy amplifier 15. The net result is the same, however, and the basic functions presented in FIG. 1 capture the essence of the operation of these spectrometers as a class.
In digital spectrometers, single baseline values B 38 are captured by the baseline capture circuit 22, and, having the same noise as the amplitude values a 37 must therefore be averaged so that their noise does not degrade the resolution of the spectrometer. U.S. Pat. Nos. 5,684,850, 5,870,051 and 5,873,054 by Warburton et al. [WARBBURTON 1997, 1999A, and 1999B] provide further details on this problem, including various methods of averaging successive baseline measurements to reduce their variance. In order to produce these baseline averages, the values captured by the baseline capture circuit 22 are fed directly to the DSP 25, where the averages are formed and the subtraction represented by subtraction circuit 23 takes place. Functionally, however, the net effect is again the same as represented by FIG. 1.
The High-Input-Count-Rate Problem
As the count rate in the detector increases, it becomes increasingly difficult to obtain an accurate baseline measurement. FIG. 2B shows the origin of the problem, namely that at high data rates the output of the energy filter circuit 17 may not return to its baseline value very often, since this requires an inter-event spacing that significantly exceeds an output pulse basewidth (i.e., xcfx84b 39 in FIG. 2A). We note in this context that xcfx84bxe2x88x921 sets the natural scale for determining whether a data rate is xe2x80x9chighxe2x80x9d for a particular energy filter, since, in a system with an optimally efficient pileup inspection circuit, xcfx84b becomes the energy filter""s deadtime xcfx84d, as will be discussed in xc2xa71 below. In such a system, provided the conversion time of the MCA is negligible (which is commonly true for digital systems and modem, high speed MCA""s as well), the output counting rate (OCR) of the spectrometer at a given input counting rate (ICR) is given by the extending dead time formula [KNOLL 1989, Chapter 4, Section VII, xe2x80x9cDead Timexe2x80x9d]:
OCR=ICRexp(xe2x88x92ICRxcfx84d),xe2x80x83xe2x80x83(1)
whose maximum occurs at ICRmax=xcfx84dxe2x88x921 with the maximum OCRmax=ICRmax/e=(e xcfx84d)xe2x88x921.
xe2x80x9cHighxe2x80x9d counting rates, then, are ICR values which approximate or exceed ICRmax and therefore scale as xcfx84dxe2x88x921 xcfx84bxe2x88x921. At this point, the majority of output pulses ([1xe2x88x921/e]=63%) are pileups that interfere with each other, and only a minority (1/e=37%) have valid peak amplitudes to be captured. FIG. 2B shows this situation schematically. The preamplifier output trace 40 now has a much higher density of event pulses. Many of these pileup in the energy filter circuit 17 output signal 42, so that only a few peak amplitudes A (e.g., 45 and 46) remain valid measures of the input signal step heights. Similarly, the opportunities to capture baseline values B 48 fall off rapidly as well as ICR values increase past ICRmax.
FIG. 3 shows a practical effect of this situation. The shown data were collected in a moderately noisy environment using an X-ray Instrumentation Associates DXP-4C digital x-ray spectrometer connected to a high quality HPGe x-ray detector exposed to an Fe-55 radiation source, where the ICR was adjusted by adjusting the source-detector distance. Data are shown for four peaking times: 1.0 xcexcs 50, 4.0 xcexcs 52, 16 xcexcs 53, and 20 xcexcs 55. In the latter three cases the point of maximum throughput ICRmax is indicated. ICRmax could not be reached with the source strength available in the case of the 1.0 xcexcs peaking time. This point would occur at about 465,000 cps. In the other three cases, we see that energy resolution begins to degrade sharply at about 1.5 times ICRmax and becomes extreme, with the spectrometer essentially failing beyond about 2.0 times ICRmax. In this regime baseline samples can no longer be gathered reliably and those that are captured are frequently contaminated with energy from a soon-to-arrive pulse which has not yet quite triggered the pileup inspection circuit. This, coupled with the circuit""s inability to track any low frequency fluctuations in baseline, sets an upper limit on acceptable circuit performance in the presence of noise. Under optimal, low background noise conditions, significantly higher counting rates can be obtained than those shown in FIG. 3 before the energy resolution begins to degrade so significantly.
Not shown in FIG. 3 is the accompanying phenomenon of centroid shift at high ICR values. Not surprisingly, since pulse amplitudes are the difference between their maxima and their baselines, these differences typically show systematic drifts from their true locations as the baselines degrade at high data rates. As a result, spectral energy peaks show the same drifts and this interferes with the ability of automatic spectral analysis software to operate properly.
In many analytical situations this problem can be dealt with simply by limiting either the count rates to which the detector is exposed or the noise environment in which the instrument operates. However, there are also many cases where neither condition is readily controlled. In x-ray experimentation, wide ranges of ICR are commonly seen in diffraction experiments, where the count rates in the strongest Bragg scatter peaks may exceed the rates in the weakest peaks and background by five or six orders of magnitude. In gamma-ray spectroscopy, extremes in counting rates are often encountered in monitoring or screening applications. If, in these situations, the counter geometry is designed to limit maximum ICR to values less than ICRmax, then typically encountered counting rates will tend to be very much less than ICRmax, requiring excessive counting times to collect data. If, on the other hand, the geometry is designed to produce reasonable counting times in typical cases, the data collected during overload conditions will be of poor or possibly unusable quality, which is unfortunate since these conditions are often of particular significance. Similarly, instruments used in the field often have no control at all over the noise environment in which they function. A method to enhance the xe2x80x9cdynamic rangexe2x80x9d of ICR""s over which a spectrometer could function without resolution degradation, even in noisy environments, would therefore be beneficial in a variety of radiation detection applications.
We also note that these problems are exacerbated when working with low energy radiation, where the energy of an event approaches the value of the preamplifier""s output noise (e.g., x-ray detectors working with x-rays below 1 keV in energy), making the situation xe2x80x9cnoisyxe2x80x9d even under ideal conditions. In these cases the energy filter is also commonly used to detect events, which significantly degrades the circuit""s ability either to do accurate pileup inspection or to assure that captured baseline events are free of energy contamination from very low energy events. A method which enhanced the efficiency of the pileup inspection circuit 18 when dealing with very low energy events would therefore also be beneficial.
The present invention provides techniques (including method and apparatus) for increasing the number of baseline samples that can be captured at high input counting rates (ICR""s), thereby allowing a radiation spectrometer to work at higher ICR""s without substantial energy resolution degradation or shifting of spectral peak features.
In brief, the approach entails providing, in addition to the energy filter, a second filter (the baseline filter), and capturing baseline values from the baseline filter. These captured baseline values are then used to estimate the energy filter""s baseline value.
More specifically, the energy filter transforms event pulses into shaped pulses, at least some of which are measured to obtain characteristic values (e.g., peak values, area values, etc.) as estimates of the energies of their associated events. An energy filter baseline value is used to compensate the characteristic values for the fact that the output of the energy filter is non-zero in the absence of event pulses.
However, rather than capturing baseline values from the energy filter, a determination is made of times when the baseline filter is at its baseline value, and during such times, baseline values are captured from the baseline filter. With knowledge of the parameters that characterize the energy and baseline filters, the baseline values captured from the baseline filter are used to update the energy filter baseline value. In specific embodiments, the energy filter and the baseline filter are linear filters, and the baseline value of the energy filter can be obtained from the baseline value of the baseline filter simply by scaling the latter by a constant, whether or not the two filters have the same functional dependence on time.
It is preferred that the baseline filter have a basewidth that is shorter than the basewidth of the energy filter so that it is at its baseline more often than the energy filter is at its baseline. In a preferred implementation, both filters are trapezoidal, with the time constants of the baseline filter being one quarter of those of the energy filter, with the resultant scaling constant being 16. By reliably providing baseline samples, this implementation preserves both energy resolution and stable peak centroids even when the detector ICR has exceeded the energy filter""s ICR of maximum throughput (ICRmax) by the same factor of four.
A typical prior art radiation spectrometer comprises two channels: a fast channel that uses a very short-basewidth pileup filter to detect pulses in the preamplifier signal and inspect them for pileup, and a slow channel that uses a longer basewidth energy filter to extract the amplitudes of these pulses. In this prior art, both shaped pulse amplitudes and baseline values are captured from the energy filter output under control of the pileup inspection circuit. As mentioned above, the present invention replaces the capture of baseline samples from the energy filter with the capture of baseline samples from the baseline filter, which typically has a basewidth that is less than that of the energy filter (but typically longer than that of the pileup filter). In some cases, the baseline filter and the pileup filter can be the same filter. This capture of baseline values from the baseline filter is also under the control of the pileup inspection circuit, appropriately modified.
Thus the invention recognizes and exploits the fact that the maximum ICR at which a filter operates effectively increases as its peaking or shaping time (and hence its basewidth) decreases, and therefore contemplates constructing a spectrometer employing a first filter (the energy filter) with a longer peaking time and basewidth, whose output is captured to provide an event energy measurement, and a second filter (the baseline filter), preferably with a significantly shorter peaking time and basewidth, from whose output baseline values can be captured and averaged to provide accurate estimates of the energy filter""s baseline value.
Embodiments can include one or a number of enhancements to optimize the invention""s performance. First, a test can be made to assure that the baseline filter is truly at baseline when capturing values from it. A simple way to do this is to require that the baseline filter output is below some predetermined threshold. However, since this method can produce biased results, in our preferred implementation we modify the pileup inspector to make this test, in addition to its job of capturing energy filter peaks that are not piled-up. The test that the baseline filter has returned to its baseline and may be validly sampled is that at least a baseline filter clearing time (which is essentially the baseline filter""s basewidth) must have elapsed since the most recent event pulse was detected. The pileup inspector accomplishes this by measuring the elapsed time following each event pulse, and waiting for the elapsed time to exceed a preset baseline filter clearing time. Whenever this happens, the baseline filter will have returned to its baseline, allowing baseline values to be validly sampled. Since the elapsed time measurement is restarted each time a new event pulse is detected, the test will fail whenever two consecutive event pulses are less than a baseline filter clearing time apart and the output of the baseline filter has not had time to return to its baseline.
Second, because the baseline filter""s basewidth is preferably significantly shorter than that of the energy filter, it is usually necessary to average multiple baseline measurements so that subtracting the baseline average from a measured energy filter peak does not significantly increase the noise in the resultant energy value.
Third, invalid baseline values arising from other, rarer forms of pileup or processing errors can be eliminated from our baseline average. This can be accomplished by collecting all baseline values into a baseline spectrum and, from time to time, determining its width at a preselected fraction of maximum (typically 5%). This width is then used to apply a cut to captured baseline values, and only those that satisfy the cut are included in the baseline average. Because the baseline spectrum is continually updated, this method also tracks the baseline if it is drifting in time.
We also present two extensions of the basic invention. In the first extension, we note that, when the baseline is drifting or reflecting some low frequency noise source, our standard baseline averaging method produces a trailing average whose value will always lag the value of the baseline at the time any particular energy filter peak value is captured. The reliable availability of baseline samples inherent in our invention allows us to address this issue by forming a baseline average comprised of measurements which are more or less symmetrically disposed in time about each captured energy filter peak. This is accomplished by placing each captured energy filter peak value into a circular buffer and subtracting from it a baseline average that is formed a fixed number of baseline samples later. We show a design that uses DSP memory as a buffer to accomplish this goal.
In the second extension, we adapt the technique to processing soft x-rays or other low energy events whose energy is too low to trigger the fast channel discriminator in a standard pileup inspector. In this case we take advantage of the fact that the noise from our middle-time-constant baseline filter is significantly lower than the noise from short-time-constant filter in a standard, fast channel pileup inspection circuit. Thus, by replacing the latter filter with the former as the input to the pulse detecting discriminator in our pileup inspection circuit, we can set the discriminator threshold to much lower energies, which allows reliable operation into the sub-kilovolt regime. At high data rates, however, there is a significant probability of capturing baseline values which pass the baseline filter clearing time test but are still contaminated by low energy pulses which have not yet triggered the pileup inspection circuit. To solve this problem, we buffer captured baseline values and have the pileup inspection also measure a post-collection clearing time. If no pulse occurs within this second inspection period, then the baseline value is valid and can be included in the baseline average. We present a spectrum taken using this method that shows a clean separation between a boron K x-ray line at 185 eV and the spectrometer""s noise at lower energies.
In applications with substantial numbers of both very soft and hard x-rays, both short and middle-time-constant filters can be connected to discriminators feeding the pileup inspection circuitry. At the cost of only a small increase in pileup inspection logic, this method combines the short-time-constant filter""s superior time resolution for eliminating hard energy x-ray pileups with the middle-time-constant filter""s superior energy resolution for reliably detecting and capturing soft x-rays into the spectrum.
While our preferred implementations are digital, digital processing techniques are not required by the invention, which could equally well be implemented using classical analog spectroscopy techniques. We discuss this approach briefly. While we present our invention primarily in the context of radiation spectroscopy using solid state detectors, the underlying methods apply equally well to other spectroscopies, including nuclear and particle spectroscopies, which use the same fundamental filtering techniques.
A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.